Custom ammunition while shifted

How do you equip custom ammunition to a shifted shape?

I gave my drider a bow in polymorph.2da and want to also equip custom arrows to the shape. I've tried the following in the module OnEquip handle:

            if( sItemTag == "rl_driderbow" )
            {
                object oArrow = CreateItemOnObject("driderarrow1",oPC,99);
                AssignCommand(oPC,ActionEquipItem(oArrow,INVENTORY_SLOT_ARROWS));
                SendMessageToPC(oPC,"Drider arrows equiped.");
            }

The code fires, it sends me the message, but the arrows never equip and if I attack something it says I'm Out of Ammo.

The bow works fine with the Unlimited Ammo property, but I want to add an OnHit property that I can't get otherwise.